Gas absorption spectroscopy is performed by measuring the percentage of light which passes through a gas sample at given light wavelengths. Particular gases exhibit characteristic light absorption responses as the wavelength of the light passing through the gas is varied. Gases can be identified by these responses.
Such identification is typically performed by identifying the presence of one or more absorption "lines" in a gas sample. An absorption line is a narrow band of light wavelengths or wavenumbers at which the gas absorbs or attenuates light. A given gas usually has a number of absorption lines at different wavenumbers.
The concentration of a target gas in a gas sample of unknown composition can be determined by the relative light absorption of the sample at wavenumbers corresponding to an absorption line of the target gas. The relationship between molecular concentration and the absorbing effect of a single absorption line is well known.
In general, the molecular concentration N of a target gas in a gas sample can be determined by making two measurements at a wavenumber corresponding to an absorption line: (1) the intensity of light transmitted through the target gas; and (2) the intensity of light transmitted through the same distance in the absence of the target gas. The ratio of these two measurements is defined as transmittance.
The success of these measurements depends on the availability of a stable source of monochromatic or narrow-bandwidth light-light having a very narrow range of wavenumbers. Optical filters or monochromators can be used to supply relatively narrow-bandwidth light. However, light produced in this way typically has a bandwidth larger than an absorption line itself. Accordingly, measurement systems using filters or monochromators typically average measurements over several or many absorption lines. Because of this, these measurement systems are subject to light absorption from interfering gases.
Lasers have been used in recent years as light sources for gas spectroscopy. Lasers produce a light beam having a very narrow frequency bandwidth. Because of this, an instrument using a laser as a light source is able to measure transmittance over a single absorption line. Such transmittance measurements are more accurate than measurements using broadband sources of light. Optical filters or monochromators are not required in order to achieve this accuracy.
However, the accuracy of gas concentration measurements remains limited by other factors. For example, calculating molecular concentration from the two measurements mentioned above requires knowledge of the "ideal" absorption characteristics of the target gas. These characteristics must be measured from a pure gas sample with a very high degree of accuracy. A further complication, however, is that such characteristics are variable with both the temperature and pressure of the gas. This means not only that the absorption response characteristics must be predicted or measured for the pure gas under varying temperature and pressure, but also that the pressure and temperature of the gas sample must be measured to obtain meaningful results. The need for such measurements adds a degree of uncertainty to the calculations, reducing accuracy.
Another limitation relates to the devices and control circuits used for measuring transmitted laser intensities. Typically, some form of radiation intensity sensor is used to measure the intensity of the laser beam after it has been transmitted through the gas sample. An electronic amplification circuit is normally associated with the sensor. In order to make accurate measurements of the transmitted light intensities, background or reference values must first be obtained from the sensor and associated electronics with the laser disabled. Such reference values represent the portion of subsequent readings which is attributable to background sources such as ambient light and amplifier biases. The reference values change with varying ambient light and temperature, as well as with sensor and amplifier drifting.
Because of the variable background values, reference measurements must be made at frequent intervals to ensure accuracy. A convenient way to make such a measurement in a laser system is to disable drive current to the laser, thus eliminating the laser beam. However, this practice causes further complications. When the drive current to the laser is interrupted the laser components immediately begin to cool. Upon resumption of the drive current a finite time is required for the intensity of the laser beam to asymptotically approach its previous operating state. During this finite time the laser frequency changes rapidly. Measurements made during this time are therefore of no value. Rather, measurements must be delayed until the laser has reached its equilibrium state. This can often take as long as or longer than the desired measurements themselves.
The constraints described above severely limit the sensitivity and accuracy of most laser-based gas measurement systems. To increase sensitivity, many such systems use one or more "White cells" to increase the effective path length of the laser beam through the sample gas. A White cell is a sample chamber which reflects the laser beam back and forth a number of times through the sample gas before an intensity measurement is performed. A disadvantage of White cells is that they increase the chances for laser fringing. They are also large and delicate, complicating optical focusing. White cells also add significant cost to a system.